Seismic Swarm Near Borrego Springs, California: Analysis of the March 2025 Event

A seismic swarm designated S20250320.2 occurred 12 km east-northeast of Borrego Springs, California, from 15:50 on 19 March 2025 to 07:19 on 26 March 2025. Over 159 hours and 29 minutes, the sequence produced 81 earthquakes. The events clustered at shallow depths, predominantly between 2 km and 11 km, with the majority occurring between 5 km and 10 km.

The largest event reached magnitude 3.7 on 20 March 2025 at 07:58:40 UTC at a depth of 10 km. Additional notable shocks included magnitudes 2.9, 2.5, 2.4, 2.3, and 2.0, all recorded within the first five days. Most events registered below magnitude 1.5, consistent with typical swarm behavior in which numerous small-magnitude quakes occur without a single dominant mainshock.

Borrego Springs lies within the tectonically active Salton Trough, a pull-apart basin formed by right-lateral strike-slip motion along the San Andreas Fault system. The immediate area is influenced by the San Jacinto Fault Zone and subsidiary structures such as the Coyote Creek Fault. These faults accommodate a significant portion of the Pacific–North American plate boundary slip rate, estimated at 15–20 mm per year in this segment. The region experiences frequent microseismicity and occasional moderate earthquakes due to its location at a complex fault junction.

Earthquake swarms in this setting often reflect fluid migration or aseismic slip triggering brittle failure on nearby fault segments. Depths recorded during the March 2025 swarm align with the brittle-ductile transition zone in the upper crust of the Salton Trough, where temperatures permit stick-slip behavior above approximately 10–12 km.

Historical records since 2000 document 13 prior swarms in the same locale, occurring in 2003, 2006, 2009 (two events), 2010 (two events), 2013, 2015, 2022 (three events), 2023, and 2024. This recurrence pattern indicates persistent strain accumulation and release along the local fault network.

The 2025 swarm concluded without producing damage or felt reports beyond the immediate desert area. Continued monitoring by regional seismic networks remains essential for understanding long-term fault behavior in this high-hazard corridor.

References

SeismoSight internal swarm classification S20250320.2
USGS Earthquake Catalog (regional fault data)
Southern California Earthquake Data Center (tectonic setting)